The use of electrotherapy by medical investigators historically reaches back to the eighteenth century. In that era, electrotherapy static generators were the subject of substantial interest. As the twentieth century was approached, experimentation applying high frequency currents to living tissue took place, d'Arsonal being considered the first to use high frequency currents therapeutically. The use of high frequency currents for the purpose of carrying out electrosurgical cutting and the like was actively promoted in the 1920s by Cushing and Bovie. In the 1970s, solid state electrosurgical generators were introduced, and a variety of such generators now are available in essentially all operating theatres.
When high frequency currents are used for cutting and coagulating, the tissue at the surgical site is subjected to controlled damage. Cutting is achieved by disrupting or ablating the tissue in immediate apposition to the excited cutting electrode, i.e., slightly spaced before it so as to confront a gap and tissue resistance combination which will support the formation of a cutting arc. Continuous sine waveforms generally are employed to carry out the cutting function where tissue cells adjacent to the electrode are vaporized. An advantage of this electrosurgical cutting procedure over the use of the cold scalpel resides both in an ease of cutting and a confinement of tissue damage to very small and shallow regions. In the latter regard, cells adjacent the cutting electrode arc are vaporized and cells only a few layers deeper are essentially undamaged. These cutting systems, in general, are employed in a monopolar manner wherein the cutting electrode is considered the active one and surgical current is returned from a large, dual component dispersive electrode coupled with the skin of the patient at a remote location.
Coagulation also may be carried out using a high frequency generator source and is accomplished by denaturation of tissue proteins due to thermal damage. Interrupted or discontinuous waveforms typically are employed to carry out coagulation. Coagulation is considered generically as including:                (1) fulguration in which tissue is carbonized by arc strikes,        (2) desiccation in which the cells are dehydrated, and        (3) white coagulation in which tissue is more slowly heated to a coagulum. The interrupted wave based coagulation procedure has been carried out with both monopolar and bipolar systems.        
In order to obtain cutting with hemostasis to arrest bleeding, present day electrosurgical generators may be controlled to blend cutting and coagulating waveforms. To achieve this blend, for instance, a lower amplitude continuous sine waveform is combined with higher amplitude coagulate pulses prior to output voltage elevation by power amplification procedures or the like.
The electrosurgical cutting reaction has been the subject of considerable study. In this regard, some investigators observed that cutting is achieved as the electrical conduction of current heats the tissue up to boiling temperatures and the cells are basically exploded as a result of the phase change. Another, parallel mechanism has been described wherein, as an intense electromagnetic field impinges on absorbing tissue, an acoustic wave is generated by the thermal elastic properties of the tissue. The origin of the pressure wave lies in the inability of the tissue to maintain thermodynamic equilibrium when rapidly heated. See generally:                “Electrosurgery” by J. A. Pierce, John Wiley & Sons New York, N.Y.        
Paramount to the cutting procedure is the generation of an arc within the evoked vapor phase. When cutting is being performed, the cutting electrode is not in mechanical contact with tissue, but rather rides on a vapor film as it is moved through the tissue. Thus, it is the separation between the cutting electrode and tissue which allows the possibility for arc formation while cutting. With the existence of this arc, current flow is highly confined, arcs by their nature being quite localized in both space and time, consisting of very short high current density discharges.
Electrosurgical generators generally are configured to derive a requisite arc formation with an active electrode of fixed geometry. For instance, the active electrodes may take the shape of a rod or spade-shaped scalpel. Arc formation requires technique on the part of the surgeon, the electrode being gradually moved toward target tissue until the spacing-based impedance is suited for striking an arc. The energy creating the arc typically is generated by a resonant inverter operating at an RF frequency. Control over such inverters is problematic, inasmuch as the arc represents a negative dynamic impedance. In general, some regulation of voltage feeding the RF inverters is carried out, however, overall output control is based upon a power level selection. Inverter control by output voltage feedback generally has been avoided due principally to the above-noted load characteristics of the necessary arc. Such attempted control usually evolves an oscillatory instability. Accordingly, power-based control is employed with marginal but medically acceptable output performance. In this regard, the environment of the arc sustaining electrode-tissue gap may change in the course of forming an incision. Upon loss of the arc, correction is made by backing the electrode away to increase or reestablish requisite tissue-gap resistance and/or by manually adjusting a generator knob to turn up its power output. However, there are limits to the latter adjustment. Should the tissue/arc resistance encountered by the generator drop excessively, to avoid excessive power generation, the generators will, in effect, turn off. This is a characteristic of all electrosurgical generators since there is a well-known relationship between output power (P), applied voltage (V) and tissue and gap resistance (R) which may be expressed as follows:P=V2/R
As resistance (R) continues to decrease voltage (V) must decrease to prevent output power (P) from increasing to such impractical or power cutoff levels to defeat an electrosurgical procedure. A somewhat common reaction to an apparently unrecoverable loss of cutting arc has been to fault the equipment and return to the procedure with replacement generators and cutting electrodes.
Currently developing electrosurgically implemented medical instrumentation often involves active cutting electrodes of highly elaborate configuration with a geometry which alters active surface areas in the course of a procedure, for example, isolating and then capturing a target lesion. One such instrument is described in U.S. Pat. No. 6,277,083 by Eggers, et al., entitled “Minimally Invasive Intact Recovery of Tissue”, issued Aug. 21, 2001. This instrument employs an expandable metal capture component supporting forwardly disposed, arc sustaining electrosurgical cutting cables. Those cutting cables, upon passing over a target lesion, carry out a pursing activity to close about the target tissue establishing a configuration sometimes referred to as a “basket”. To initially position the forward tip of the involved instrument in confronting adjacency apposite the targeted tissue, an assembly referred to as a “precursor electrode” is employed. In the latter regard, the forwardmost portion of the instrument tip supports the precursor electrode assembly. That electrode assembly is initially positioned within a small incision at the commencement of the procedure, whereupon it is electrosurgically excited and the instrument tip then is advanced to a target confronting position.
An improved design for the instrument, now marketed under the trade designation “en-bloc” by Neothermia Corporation of Natick, Mass., is described in co-pending application for United States patent by Eggers, et al., entitled “Minimally Invasive Intact Recovery of Tissue”, Ser. No. 09/904,396, filed Jul. 12, 2001 and assigned in common herewith now U.S. Pat. No. 6,471,659, issued Oct. 29, 2002. To accommodate for the arc-to-tissue resistance variations encountered by an electrosurgical generator in driving the dynamically altering cutting surface, an improved electrosurgical generator was developed by Eggers, et al. Described in application for U.S. patent application Ser. No. 09/904,412 entitled “Electrosurgical Generator”, filed Jul. 12, 2001, now U.S. Pat. No. 6,740,079, issued May 25, 2004, and assigned in common herewith, the generator exhibits constant voltage and variable power attributes addressing the requirement for sustaining an arc at a dynamic electrode assembly. The generator design also recognizes the operational aspect of initially creating or “striking” an arc both at the precursor electrode assembly and at the capture component cutting cables at the outset of a procedure. At this initial part of a procedure, the electrodes will be embedded or in direct contact with tissue. The conventional surgical technique of spacing the cutting electrode from tissue to start an arc thus is not a practical approach to arc formation. To create an arc at procedure commencement or restart, the generator elevates a control voltage to an extent effecting arc creation at an elevated power level for a boost interval of time which is relatively short but heretofore elected to assure arc creation. For example, the enabling boost control signal has been sustained for 375 milliseconds. The generator is marketed as a “Model 3000 Controller” by Neothermia Corporation (supra).
Studies also have revealed that the electrical resistance characteristics encountered by electrosurgical generators and their associated instruments will vary quite widely in dependence upon the resistivity characteristics of involved tissue. Accordingly, for given electrosurgically based systems, optimization of the power vs. resistance profile is called for to avoid loss of arc on one hand, and to avoid tissue specimen damage due to excessive power application on the other hand.
Surgical procedures, including those described above, are increasingly being performed using local anesthesia in place of general anesthesia with the benefit of shorter post-surgery recovery time, shorter hospital stay, lower risks to patients associated with general (total body) anesthesia and lower associated procedure and/or hospitalization costs. Local anesthetic agents are weakly basic tertiary amines, which are manufactured as chloride salts. The molecules are amphipathic, and have the function of the agents and their pharmacokinetic behavior can be explained by the structure of the molecule. Each local anesthetic has a lipophilic side; a hydrophilic-ionic side; an intermediate chain, and, within the connecting chain, a bond. That bond determines the chemical classification of the agents into esters and amides. It also determines the pathway for metabolism. Local anesthesia is commonly administered (1) in the spine (caudal and epidural anesthesia), (2) between the ribs (inter costal anesthesia), (3) into the dental pulp (intra pulpal), (4) intravenous regional anesthesia (where a tourniquet is used to prevent anesthetic from entering systemic circulation, Bier block), (5) regionally injected anesthetic which forms “walls” of anesthesia encircling the operative field (field block) and (6) highly localized injection of the anesthetic close to the nerves located within the operative field (nerve block). In each of these approaches, the active anesthetic drug is administered for the purposes of intentionally interrupting neural function and thereby providing pain relief.
A variety of local anesthetics have been developed, the first agent for this purpose being cocaine which was introduced at the end of the nineteenth century. Lidocaine is the first amide local anesthetic and the local anesthetic agent with the most versatility and thus popularity. It has intermediate potency, toxicity, onset, and duration, and it can be used for virtually any local anesthetic application. Because of its widespread use, more knowledge is available about metabolic pathways than of any other agent. Similarly, toxicity is well known.
Vasoconstrictors have been employed with the local anesthetics. In this regard, epinephrine has been added to local anesthetic solutions for a variety of reasons throughout most of the twentieth century to alter the outcome of conduction blockaid. Its use in conjunction with infiltration anesthesia consistently results in lower plasma levels of the agent. See generally:                “Clinical Pharmacology of Local Anesthetics” by Tetzlaff, J. E., Butterworth-Heinemann, Woburn, Mass. 2000        
To minimize the possibility of irreversible nerve injury in the course of using local anesthetics, the drugs necessarily are diluted. By way of example, the commonly used anesthetic drug is injected intramuscularly to effect a nerve block or field block using concentrations typically in the range of 0.4% to 2.0% (weight percent). The diluent contains 0.9% sodium chloride. Such isotonic saline is used as the diluent due to the fact that its osmolarity at normal body temperature (for example 37° C.) is 286 milliOsmols/liter which is close to that of cellular fluids and plasma which have an osmolarity of 310 milliOsmols/liter. As a result, the osmotic pressure developed across the semipermeable cell membranes is minimal when isotonic saline is injected intramuscularly and extracellularly. Consequently, there is no injury to the tissue's cells surrounded by this diluent since there is no significant gradient which can cause fluids to either enter or leave the cells surrounded by the diluent. It is generally accepted that diluents having an osmolarity in the range 240 to 340 milliOsmols/liter are isotonic solutions and therefore can be safely injected intramuscularly.